It has been a problem to provide a procedure that is capable of transmitting, in a secured way, numerical data that correspond to high quality images in any numerical format, from an encoding in wavelets, live or pre-recorded to a visualization screen and/or for being recorded on a hard disk or a completely different back-up device belonging to a box that connects a remote transmission network to a display of the monitor type or a television screen, all while preserving visual quality, but avoiding fraudulent utilization such as the possibility of making pirated copies of numerically encoded images and recordings on a back-up support of the decoder box.
The traditional techniques of cryptography consist in general of combining (according to operations such as addition or subtraction) the data of a source with values generated in a pseudo-random way and from an initialization key (encryption key). Simple possession of the key thus allows the complete decryption of the encrypted data, those that contain in substance the totality of the original information.
EP 1 011 269 A1 discloses a method for encryption of an information signal that can be applied in the case of fixed images. The method adds to an original signal that is not compressed a pseudo-random noise to obtain a new signal. The signal encrypted in this way is next compressed with the aid of standard, adequate algorithms, then transmitted. The key itself is transmitted, in a secure manner, to the future user of the encrypted signal. The authors indicate that such a method can be applied in the case of encoded images of the standard JPEG. No reference is made to the case of images encoded by wavelets. Moreover, possession of the key conditions decryption of the transmitted signal.
EP 0 920 209 A1 discloses two processes of scrambling as well as their associated descrambling procedures to protect video data encoded according to standard MPEG-2. These processes act on at least a block of DC and AC coefficients resulting from the transformation DCT and for at least an image intra I. According to the first procedure, collection of coefficients AC of at least a block of coefficients (DC, AC) of at least an image I, undergo a permutation of order R. The associated descrambling procedure consists of transmitting the parameter R to execute the inverse and rebuild the original MPEG-2 stream. The collection of original data is therefore always present in the protected stream. According to the second procedure, coefficient DC of at least a block of coefficients (DC, AC) of at least an image I, is recalculated with the aid of an action command that could be random. Nevertheless, for the descrambling, all the data of the reconstitution of the aforementioned original stream (the order R, the original coefficients DC encrypted with the aid of a key), in such a way that all the data that constitute it are comprised in the protected MPEG-2 stream and dispatched together and through the same route, the protected MPEG-2 stream that contains the information for the reconstruction being the unique output data of the scrambling process. Consequently, this former method does not satisfy the criteria of high security.
Another reference is Zing W et al., published in the ACM Multimedia Proceedings of the International Conference on Oct. 1999 and titled “Efficient Frequency Domain Video Scrambling for Content Access Control.” That article discloses a method to protect the numerical data that encode multimedia content. The method is based on a collection of three basic pseudo-random operations (bit inversion, permutation and rotation of the block of coefficients) that can be combined and controlled by the encryption keys. The content collection resides in the protected stream and can be accessed by encryption keys that will be transmitted to authorized users. In this former method, collection of original data are present in the protected stream and access to the original content is completely conditioned upon possession, or not, of encryption keys. This former method thus does not solve the security problem.